Legal and practical obstacles make negative rates unlikely, but Janet Yellen suggests such a move is possible

Federal Reserve Chairwoman Janet Yellen waded into fraught territory before Congress, suggesting the central bank could turn to negative interest rates in an economic downturn despite legal and other uncertainties.

Her comments Wednesday came as concerns about unsettled markets and weak global growth pushed benchmark U.S. Treasury rates to a one-year low. The yield on the 10-year note fell to 1.706%, leaving it down more than half a...